CC is on the psp. its understandable for it to not be as grand as a normal, fully numbered, stand alone final fantasy.
FF XIII just didnt deliver on what ppl have come to expect from a fully numbered, console FF. its that simple.
then throw in the "extras" like FFXIII taking itself too seriously, Zack being a more likable/relatable character, an arguably more fun combat system in CC, a relatively coherent story, and you have all the reasons you'd need to understand why, apparently, CC was praised so much
side note: no one ever praises a game BECAUSE of its linearity. if CC was indeed held in righ regards, its because of everything else DESPITE being linear. ff13 didnt have much to compensate for being linear and boring